1. Field of the Invention
The present invention relates to a mobile communication device such as a personal handy phone, a mobile telephone, a pager, etc. and, more particularly, a direct converting receiver to which a direct conversion system is applied.
2. Description of the Prior Art
In recent years, radio communication systems such as a personal handy phone, a mobile telephone, a radio paging device (pager), a cordless telephone, etc., which are employed mainly in mobile communication, have been drastically spread. As a receiver system of these radio communication systems, a direct converting receiver whose configuration is ready to be integrated and is suitable for miniaturization and light weight has been known.
As the direct converting receiver in the prior art, a configuration as disclosed in Unexamined Patent Publication Hei 7-135514, for example, has been known. An example of a receiver circuit of the direct converting receiver in the prior art is shown in FIG. 17. FIG. 17 shows a configuration of the receiver circuit to which frequency shift keying (abbreviated as xe2x80x9cFSKxe2x80x9d hereinafter) is applied. The configuration and an operation of the receiver circuit will be explained hereinafter.
The direct converting receiver comprises a receiving antenna 101 for receiving a radio signal, a signal attenuator (ATT) 102 for attenuating a received signal, a received signal amplifier 104 for amplifying the received signal supplied from the signal attenuator 102 and outputting it as a modulated signal 103, a local oscillator 106 for generating a local signal 105, a 90-degree phase shifter 107 for shifting a phase of the local signal 105 by 90 degree, a first signal mixer 108 for mixing the modulated signal 103 with the local signal 105, and a second signal mixer 109 for mixing the modulated signal 103 with the local signal 105 which has been subjected to 90 degree phase shift.
Further, an I low-pass filter 111 and a Q low-pass filter 113 are provided in the direct converting receiver. The I low-pass filter 111 can pass only a baseband signal which is common in phase with the received signal, which is contained in an output signal of the first signal mixer 108, (i.e., an I baseband signal from which harmonic components are removed) to then output an I baseband signal 110. The Q low-pass filter 113 can pass only a baseband signal which is obtained by phase-shifting the received signal, which is contained in an output signal of the second signal mixer 109, by 90 degree (i.e., a Q baseband signal from which harmonic components are removed) to then output a Q baseband signal 112 which is intersected orthogonally with the I baseband signal 110. These low-pass filters 111, 113 are composed of filters which have a fixed cut-off frequency, i.e., whose cut-off frequency is not varied, respectively.
Furthermore, there are provided a demodulating means 115 and an electric field strength detecting means 116. The demodulating means 115 can receive the I baseband signal 110 and the Q baseband signal 112, then demodulate them, and then output a demodulated signal 114. The electric field strength detecting means 116 can receive the I baseband signal 110 to detect a level of the received signal, and then output a control signal to the signal attenuator 102.
Next, a receiving operation of the direct converting receiver constructed as above in the prior art will be explained hereunder. First, the radio signal being received by the receiving antenna 101 is input into the signal attenuator 102 which can control the level of the received signal in compliance with the control signal supplied from the electric field strength detecting means 116. The output signal of the signal attenuator 102 is amplified by the received signal amplifier 104, and then output to the first signal mixer 108 and the second signal mixer 109 as the modulated signal 103 respectively. In the meanwhile, the local signal 105 being generated from the local oscillator 106 is mixed with the modulated signal 103 in the first signal mixer 108 and then output to the I low-pass filter 111. At the same time, a phase of the local signal 105 is shifted by 90 degree by the 90-degree phase shifter 107, then the local signal 105 is mixed with the modulated signal 103 in the second signal mixer 109 and then output to the Q low-pass filter 113.
Then, the I baseband signal 110 and the Q baseband signal 112 derived after they have been passed through the I low-pass filter 111 and the Q low-pass filter 113 respectively are output to the demodulating means 115 and then demodulated there to produce the demodulated signal 114. Also, the electric field strength detecting means. 116, when receives the I baseband signal 110, detects a level of the received signal and then outputs the control signal, which is in proportion to the level of the received signal, to the signal attenuator 102.
In the radio receiver, if other radio waves in addition to the desired received wave to be received are caught as the interference waves by the receiving antenna simultaneously, particularly if a plurality of interference waves whose signal levels are high to such extent that they exert an influence upon the receiving bandwidth are caught, there has been such a problem that distortion due to the intermodulation is caused between these received radio wave signals and thus receiving sensitivity of the desired received waves to be received (desired signal) is degraded.
In order to improve the above problem, the technology has been proposed wherein generation of the distortion in the amplifier or the signal mixer can be suppressed by controlling a gain of a receiver system in response to the level of the received signal, which is generally called AGC (Automatic Gain Control), to thus improve the receiving sensitivity of the desired signal to be received. If this technology is applied to the configuration in FIG. 17, an AGC means can be so constructed that the electric field strength detecting means 116 outputs the control signal to the signal attenuator 102, which is positioned immediately after the antenna, in response to the level of the received signal, and then the signal attenuator 102 controls the level of the received signal received by the receiving antenna 101 based on this control signal. According to such AGC means, it is possible to suppress the distortion due to the intermodulation caused in the received signal amplifier 104, the first signal mixer 108, and the second signal mixer 109.
In recent, the demand for the direct converting receiver which is suitable for IC integration has been enhanced with the drastic spread of the radio communication systems such as the mobile telephone, etc., and a communication capacity for mobile communication has been increased abruptly with increase of the user of the mobile communication. In answer to the increase in the demand for such mobile communication, there has been a need to enhance a transmission speed in the radio communication and to proceed a narrower bandwidth of the communication frequency.
In the event that low-pass cut-off frequencies of the I low-pass filter and the Q low-pass filter are set lower so as to take preference of reduction in the influence of the interference wave by the radio waves in adjacent channels (i.e., adjacent waves) when the bandwidth of the direct converting receiver is made narrower, such a problem has arisen that, if the oscillation frequency of the local oscillator has been widely offset and deviated from the carrier frequency of the received waves, desired components of the received signal are out of the passbands of the I low-pass filter and the Q low-pass filter and thus such desired components are eliminated and not to be demodulated.
In the direct converting receiver in the prior art, since normally respective cut-off frequencies of the I low-pass filter and the Q low-pass filter are fixed, there has been such a necessity that the configuration in which low-frequency cut-off frequencies of the I low-pass filter and the Q low-pass filter are set slightly higher at the sacrifice of adjacent wave cut-off characteristics to overcome the above problem must be employed.
The present invention has been made in light of the above circumstances, and it is an object of the present invention to provide a direct converting receiver which is capable of reducing an influence of an adjacent wave by varying a cut-off frequency of a low-pass filter in response to a signal level of the adjacent wave, and improving a tolerance for frequency offset of an oscillation frequency of a local signal from a carrier frequency of a modulated signal to be received.
In order to achieve the above object, according to first aspect of the present invention, a direct converting receiver comprises a local oscillator for generating a local signal; a first signal mixer for mixing the local signal with a modulated signal based on a received signal; a second signal mixer for mixing the modulated signal with a signal which is obtained by phase-shifting the local signal by 90 degree; an I low-pass filter for eliminating a high frequency component of an output signal of the first signal mixer; an Q low-pass filter for eliminating a high frequency component of an output signal of the second signal mixer; a demodulating means for generating a demodulated signal of the received signal by processing an I baseband signal based on an output of the I low-pass filter and a Q baseband signal based on an output of the Q low-pass filter; an adjacent wave detecting means for detecting a signal level of the received signal in a plurality of different frequency bands based on at least one of outputs of the I low-pass filter and the Q low-pass filter and then detecting a signal level of the adjacent wave; and a baseband filter controlling means for variably controlling the cut-off frequencies of the I low-pass filter and the Q low-pass filter according to an output signal of the adjacent wave detecting means.
In the direct converting receiver, if the cut-off frequencies of the I low-pass filter and the Q low-pass filter are lowered when an influence degree of the adjacent waves is large based on the signal level of the adjacent waves detected by the adjacent wave detecting means, e.g., the presence of the adjacent waves, the influence of the adjacent waves can be reduced and deterioration in the receiving sensitivity can be reduced. While, if the cut-off frequencies of the I low-pass filter and the Q low-pass filter are raised when the influence degree of the adjacent waves is small, the tolerance for the frequency offset of the oscillation frequency of the local oscillator, which is decided by the cut-off frequencies of the low-pass filters, with respect to the carrier frequency of the received signal can be enhanced. As a result, the receiving sensitivity can be improved and interference by the adjacent channels can be reduced, and also the tolerance for the frequency offset of the oscillation frequency of the local oscillator relative to the carrier frequency of the received signal can be improved.
According to second aspect of the present invention, a direct converting receiver comprises a local oscillator for generating a local signal; a first signal mixer for mixing a modulated signal based on a received signal with the local signal; a second signal mixer for mixing the modulated signal with a signal which is obtained by phase-shifting the local signal by 90 degree; a first I low-pass filter for extracting an I baseband signal by eliminating a high frequency component of an output signal of the first signal mixer; a second I low-pass filter having a cut-off frequency which is different from that of the first I low-pass filter; a first Q low-pass filter for extracting a Q baseband signal by eliminating a high frequency component of an output signal of the second signal mixer; a second Q low-pass filter having a cut-off frequency which is different from that of the first Q low-pass filter; a demodulating means for generating a demodulated signal of the received signal by processing the I baseband signal and the Q baseband signal; a first electric field strength detecting means for detecting a signal level of the received signal based on at least one of outputs of the first I low-pass filter and the first Q low-pass filter; a second electric field strength detecting means for detecting a signal level of the received signal based on at least one of outputs of the second I low-pass filter and the second Q low-pass filter; an adjacent wave detecting means for detecting a signal level of the adjacent wave based on outputs of the first electric field strength detecting means and the second electric field strength detecting means; and a baseband filter controlling means for variably controlling the cut-off frequencies of the first I low-pass filter and the first Q low-pass filter according to an output signal of the adjacent wave detecting means.
In the direct converting receiver, if the cut-off frequencies of the I low-pass filter and the Q low-pass filter are changed based on the signal level of the adjacent waves detected by the adjacent wave detecting means, e.g., the presence of the adjacent waves, the interference by the adjacent channels can be reduced, the deterioration in the receiving sensitivity can be prevented and the receiving sensitivity can be improved, and also the tolerance for the frequency offset of the oscillation frequency of the local oscillator relative to the carrier frequency of the received signal can be improved.
In a direct converting receiver, the first electric field strength detecting means receives outputs of the first I low-pass filter and the first Q low-pass filter and then detects the signal level of the received signal, and the second electric field strength detecting means receives outputs of the second I low-pass filter and the second Q low-pass filter and then detects the signal level of the received signal.
Since the first electric field strength detecting means and the second electric field strength detecting means receives the outputs of the I low-pass filter and the Q low-pass filter and then detects the signal level of the received signal, a detection efficiency can be improved in detecting the electric field strength.
In a direct converting receiver, the adjacent wave detecting means receives outputs of the first electric field strength detecting means and the second electric field strength detecting means, and then generates a signal indicating the magnitude of the adjacent wave by comparing signal levels of these input signals.
The magnitude of the adjacent waves, e.g., a signal indicating whether or not the adjacent waves are contained can be detected by comparing two signal levels by using the adjacent wave detecting means based on the outputs of the first electric field strength detecting means and the second electric field strength detecting means, and the cut-off frequencies of the first I low-pass filter and the first Q low-pass filter can be changed according to this signal. As a result, the influence of the adjacent waves can be reduced, and also the tolerance for the frequency offset of the oscillation frequency of the local oscillator relative to the carrier frequency of the received signal can be enhanced.
In a direct converting receiver, the first I low-pass filter and the first Q low-pass filter have a cut-off frequency modifying means respectively, and the baseband filter controlling means outputs a filter controlling signal, which variably controls the cut-off frequencies of the first I low-pass filter and the first Q low-pass filter, based on the output signal of the adjacent wave detecting means, and then changes cut-off frequencies of the first I low-pass filter and the first Q low-pass filter discretely or continuously according to the filter controlling signal.
The filter controlling signal can be output from the baseband filter controlling means to control the cut-off frequency modifying means for the first I low-pass filter and the first Q low-pass filter, so that the cut-off frequencies of these low-pass filters can be changed discretely or continuously. As a result, the influence of the adjacent waves can be reduced, and also the tolerance for the frequency offset of the oscillation frequency of the local oscillator relative to the carrier frequency of the received signal can be enhanced.
According to third aspect of the present invention, a direct converting receiver comprises a local oscillator for generating a local signal; a first signal mixer for mixing a modulated signal based on a received signal with the local signal; a second signal mixer for mixing the modulated signal with a signal which is obtained by phase-shifting the local signal by 90 degree; an I low-pass filter for extracting an I baseband signal by eliminating a high frequency component of an output signal of the first signal mixer; a Q low-pass filter for extracting a Q baseband signal by eliminating a high frequency component of an output signal of the second signal mixer; a demodulating means for generating a demodulated signal of the received signal by processing the I baseband signal and the Q baseband signal; an electric field strength detecting means for detecting a signal level of the received signal based on at least one of outputs of the I low-pass filter and the Q low-pass filter; an electric field strength holding means for holding signal levels detected by the electric field strength detecting means under a condition that the I low-pass filter and the Q low-pass filter are set to have at least two different cut-off frequencies; an adjacent wave detecting means for detecting a signal level of the adjacent wave based on an output of the electric field strength holding means; and a baseband filter controlling means for variably controlling cut-off frequencies of the I low-pass filter and the Q low-pass filter according to an output signal of the adjacent wave detecting means.
In the direct converting, if the cut-off frequencies of the I low-pass filter and the Q low-pass filter are changed based on the signal level of the adjacent waves detected by the adjacent wave detecting means, e.g., the presence of the adjacent waves, the interference by the adjacent channels can be reduced, the deterioration in the receiving sensitivity can be prevented and the receiving sensitivity can be improved, and also the tolerance for the frequency offset of the oscillation frequency of the local oscillator relative to the carrier frequency of the received signal can be improved.
A direct converting further comprises an operation controlling means for controlling a setting of the cut-off frequencies of the I low-pass filter and the Q low-pass filter when an electric field strength is detected by the electric field strength detecting means and also a signal holding timing in the electric field strength holding means.
In the direct converting, the signal level of the received signal in a plurality of different frequency bands can be detected by controlling the electric field strength detecting means and the electric field strength detecting means by using the operation controlling means and then the magnitude of the adjacent waves can be detected based on the signal level, so that the cut-off frequencies of the I low-pass filter and the Q low-pass filter can be according to the magnitude of the adjacent waves. As a result, the influence of the adjacent waves can be reduced, and also the tolerance for the frequency offset of the oscillation frequency of the local oscillator relative to the carrier frequency of the received signal can be enhanced.
In a direct converting, the electric field strength detecting means receives outputs of the I low-pass filter and the Q low-pass filter, and then detects a signal level of the received signal.
Since the first electric field strength detecting means and the second electric field strength detecting means receives the outputs of the I low-pass filter and the Q low-pass filter and then detects the signal level of the received signal, a detection efficiency can be improved in detecting the electric field strength.
In a direct converting, the adjacent wave detecting means receives at least two outputs of the electric field strength holding means and then generates a signal indicating a magnitude of the adjacent wave by comparing signal levels of input signals in these different frequency bands.
The magnitude of the adjacent waves, e.g., a signal indicating whether or not the adjacent waves are contained can be detected by comparing signal levels in different frequency bands by using the adjacent wave detecting means based on at least two outputs from the electric field strength detecting means, and the cut-off frequencies of the I low-pass filter and the Q low-pass filter can be changed according to this signal. As a result, the influence of the adjacent waves can be reduced, and also the tolerance for the frequency offset of the oscillation frequency of the local oscillator relative to the carrier frequency of the received signal can be enhanced.
In a direct converting, the I low-pass filter and the Q low-pass filter have a cut-off frequency modifying means respectively, and the baseband filter controlling means outputs a filter controlling signal, which variably controls cut-off frequencies of the I low-pass filter and the Q low-pass filter, based on the output signal of the adjacent wave detecting means, and then changes the cut-off frequencies of the I low-pass filter and the Q low-pass filter discretely or continuously according to the filter controlling signal.
The filter controlling signal can be output from the baseband filter controlling means to control the cut-off frequency modifying means for the I low-pass filter and the Q low-pass filter, so that the cut-off frequencies of these low-pass filters can be changed discretely or continuously. As a result, the influence of the adjacent waves can be reduced, and also the tolerance for the frequency offset of the oscillation frequency of the local oscillator relative to the carrier frequency of the received signal can be enhanced.